Open cast mining operations are well known and are conducted in a number of countries around the world. Typically they comprise progressively mining domains of an ore body in a staged batch-like process. Each so called batch comprises selectively placing explosives in the rock of the batch. Thereafter the rock is blasted to break and loosen the rock and form a muck pile. Typically the deposits in these mines are heterogeneous in the sense that the ore is disseminated in complex shaped volumes of varying grade within a host rock which is waste. The shape of each ore zone on a horizontal plane is represented by a polygon when viewed in plan.
The rock body, for example, might comprise one or more ore polygons that are economic to recover and waste rock that is to be discarded. The ore is selectively removed from the muck pile and sent to a concentrator where the valuable mineral is extracted by an appropriate technique. Similarly, the waste rock is removed and sent to a discard rock dump. Clearly an important part of this process is the accurate delineation and identification of the boundaries between high grade ore and low grade ore and between ore and waste. A mixture of scientific know-how, geology, computer modelling, and experience is used to determine the boundaries in the body of rock prior to blasting being conducted. This art has developed to the point where mining engineers and geologists have a good three dimensional picture of the boundaries between the different ores in the virgin rock prior to blasting.
However, blasting of the rock body causes some expansion of the rock body. In addition there may be some differences in the amount of movement of the different parts of the rock body.
Mining engineers and geologists sometimes work on the assumption that the ore boundaries of the blasted rock body are the same as that for the rock body prior to blasting and direct the broken rock to the concentrator and the dump respectively on this basis.
The problem is that the rock and therefore also the ore boundaries do move. Accordingly, if this movement is not accounted for by the mining engineers in the mining operation some of the desirable ore is directed to the dump. This leads to a loss of product which is intended to be recovered. Similarly, some of the waste is recovered in the ore stream and is sent to the concentrator. This can lead to a significant loss of efficiency in the concentrators as it processes more waste and less product. This can lead to a drop off in the volume of concentrate produced per unit time.
It is universally recognised that this approach is unsatisfactory. It would therefore be highly desirable if a way could be devised of measuring the movement of the rock and thereby the ore boundaries. It would enable a three dimensional picture of the ore boundaries in the pre-blast rock body to be adjusted to account for the measured rock movement. This in turn would improve the correct reporting of the ore to the concentrator and the waste to the dump.
A method for the determination of movement of a boundary between ores of different grades, or between product and waste rock, has been described in Australian Patent Application No. 2004202247. A monitor for use in such a method has also been described in that document. The monitor described includes a transmitter that is located within a casing. The casing is in turn located within an outer housing, the casing being capable of movement within and relative to the housing. While this arrangement has been found to provide some advantages in many instances, it has been found to be unsuitable in some circumstances.